Different types of calenders are used for improving the smoothness and surface profile of manufactured sheet of paper or board. One of the concurrent calender types is the soft-nip calender comprising at least two calender nips operating in succession along the sheet travel, whereby each nip is formed by a soft roll and a hard roll mounted to rotate on each other. Today, the soft roll is generally surfaced with a polymer coating, while the hard roll is a heatable roll made from cast iron. The different types of rolls are mounted as an alternating succession in a vertical stack thus forming successive nips, whereby either side of a running web travels alternately over a soft roll, a hard roll and so on, thus making both sides of the sheet maximally equal after the surface-treatment. The calender rolls, particularly the soft roll, undergo wear during the use, thereby invoking a need of scheduled replacement. Today, two different techniques of roll replacement are used. In one arrangement, the old roll with its bearing housings is elevated away from its operating position by means of an overhead hoist. Herein, either the upper roll must always be removed before the lower roll can be replaced or, alternatively, the roll stacks must be askewed from a vertical plane in order to facilitate a sideways obliquely performed lifting of the lower roll away from its normal position under the upper roll. Also in vertically aligned roll stacks it is possible to implement the removal of the lower roll to take place in a sideways direction by first shifting the lower roll laterally away from under the upper roll. In this type of a construction, the frame of the calender stack must be open at least in the direction of the lower roll removal.
When the construction is such as to allow the lower roll to be removed only after the removal of the upper roll, the roll replacement operation becomes extremely clumsy, particularly if the upper roll is a heatable roll, as is the case inevitably always for the second nip, because the roll connections such as those of the heating medium circulation must be disconnected during the removal of the roll. In a roll replacement system with a sideways shifting arrangement of the rolls, sufficient free space must be reserved for the movement of either roll. Such servicing space for roll replacement requires more footprint about the calender. As the roll diameters in modern papermaking machines are large, the headroom for roll replacement may be as large as two meters per roll and, since a calender always has at least two calender nips, the need of lateral footprint may be up to four meters for a two-nip calender. Obviously, this kind of roll mounting is not possible in such machinery rebuild operations wherein a soft-nip calender must be fitted to replace an outdated machine calender. During machinery rebuild, it may be necessary to relocate various units of the papermaking machinery and increase the length of the machine, which is expensive. Also in new factory projects, a machine of a larger overall length increases costs due to larger footprint, among other factors. Another drawback of a large lateral roll change space is that the web must travel as open draw over the roll change space, because this portion of machinery cannot be equipped with auxiliary devices. Long, open web draws increase the risk of web breaks and complicate web tail threading.
It is also possible to replace the lower roll of a calendar nip by way of elevating the upper roll apart from the lower roll and then moving the lower roll with its bearing housings aside supported by a roll transfer carriage, whereupon the roll can be replaced. This arrangement is hampered by the large lateral space required about the roll and its need for a dual set of roll handling equipment, whereby the lift must be complemented with at least two transfer carriages, which makes this construction costly.
Attempts have been made to reduce the space requirement of the calender in the machine direction of the web travel by way of, e.g., locating the calender frames of two successive roll nips, the frames having one open side, in a back-to-back disposition of the frames by their closed sides, whereby the web travel between the successive nips is maximally minimized. While this arrangement needs a smaller layout footprint, a problem arises from the roll replacement operations that now must be performed on opposite sides of the calender frame thus still needing as much roll change headroom as in any other conventional calender.